displaying a 512 *512 8 bpp image

I am having two lena image

1. lena256.bmp---> a 256 *256 image

close all; 
 clear all;
 clc;
 %--------Displaying Lena image which has 256*256 size 
imData=imread('lena256.bmp');
imshow(imData);
figure(1);
title('Original image');
pause(2);

By using above code I am able to display the image.

2. lena_2.bmp----> 512 * 512 image

close all; 
 clear all;
 clc;
 %--------Displaying Lena image which has 256*256 size 
imData=imread('lena_2.bmp');
imshow(imData);
figure(1);
title('Original image');
pause(2);

by using the above code, the image displaying is as follow

why is it so?? why second image is not correct

Answer 1

[DGM]

The image is an indexed image. You need the corresponding color map in order to do anything with it.

[A map] = imread('lena.bmp'); % read the image and the map

imshow(A,map); % use the map to display or write the image

In this case, the copy of lena.bmp appears to be a color image with a relatively short color table and no dithering. Be aware that there are many versions of this image floating around. Some are I/RGB; some are indexed grayscale or color with varying map lengths and dithering methods. Just because you have a file called 'lena.bmp' doesn't mean it's the same as something you see described in a code example on a forum somewhere.